1. Field of the Invention
The present invention relates to a fuel supply apparatus for an internal combustion engine having only an in-cylinder injector for injecting fuel into a cylinder, or an internal combustion engine having the in-cylinder injector and an intake manifold injector for injecting fuel into an intake manifold or an intake port. More particularly, the present invention relates to a technique for suppressing formation of deposits in the injection hole of the in-cylinder injector.
2. Description of the Background Art
A fuel injection apparatus for an internal combustion engine provided with an in-cylinder injector and an intake manifold injector and controlling a fuel injection ratio between the two injectors is described, e.g., in Japanese Patent Laying-Open No. 07-103048.
In the apparatus described in Japanese Patent Laying-Open No. 07-103048, the in-cylinder injectors corresponding to the respective cylinders are connected to a common fuel delivery pipe. The fuel delivery pipe is connected to a high-pressure fuel pump driven by the internal combustion engine, via a check valve allowing flow toward the fuel delivery pipe. The high-pressure fuel pipe is provided with an electromagnetic valve, and the opening/closing timings of the electromagnetic valve are controlled to adjust the quantity of the fuel discharged from the high-pressure fuel pump.
In this conventional apparatus, during the high load operation where the degree of press down of the accelerator pedal that is the load of the internal combustion engine is greater than a prescribed reference value, the fuel is injected from only the intake manifold injector toward the intake port in an early stage of the intake stroke. During this high load operation, the electromagnetic valve remains fully open, so as to stop the fuel supply from the high-pressure fuel pump to the fuel delivery pipe.
There is also known an internal combustion engine provided with an intake manifold injector for injecting fuel into an intake manifold and an in-cylinder injector for constantly injecting fuel into a combustion chamber, wherein fuel injection from the intake manifold injector is stopped when the engine load is lower than a preset load and the fuel is injected from the intake manifold injector when the engine load is greater than the preset load. In this internal combustion engine, the total injection quantity corresponding to the sum of the fuel injected from the both injectors is predetermined as a function of the load of the internal combustion engine, and the total injection quantity is increased as the load of the internal combustion engine is greater.
The in-cylinder injector is mounted such that its injection hole is directly open to the combustion chamber of the internal combustion engine. It injects the fuel having been pressurized by a fuel pump directly into the cylinder. The in-cylinder injector directly injecting the fuel into the cylinder of the internal combustion engine is configured to inject the fuel in the later stage of the compression stroke for high-precision control of the mixed state of the air-fuel mixture within the cylinder, so as to improve fuel efficiency and the like. In the in-cylinder injector thus injecting the fuel directly into the cylinder, the fuel pressure is high, which causes noise and vibration in the high-pressure fuel system. Particularly, during the low load operation (e.g., idling), the internal combustion engine makes only a small noise, making the noise and vibration from the high-pressure fuel system noticeable.
Further, since the in-cylinder injector is placed such that its tip end (where the injection hole for injecting the fuel is provided) protrudes into the combustion chamber, deposits are likely to accumulate in the injection hole, which may cause improper fuel injection. More specifically, the injection hole at the tip end of the in-cylinder injector is located in the combustion chamber, and deposits may accumulate in the injection hole in the high-temperature atmosphere, hindering injection of the fuel in a desired quantity. The temperature at the tip end of the in-cylinder injector is greatly affected by the heat received from the combustion gas. It also suffers heat received from the cylinder head, heat dissipated to the fuel, and others. It is considered that the deposits will gradually narrow the injection hole as the temperature increases.
Japanese Patent Laying-Open No. 09-021369 discloses a fuel injection control apparatus for an internal combustion engine having such a high-pressure fuel supply system, aiming at stabilization of combustion in the low load operation. The fuel injection control apparatus for the internal combustion engine includes fuel pressurizing means for pressurizing fuel, fuel injection means for injecting the fuel pressurized by the fuel pressurizing means in a controlled manner by way of opening/closing of a valve, external load detecting means for detecting a load externally applied to the engine, and pressure modifying means for decreasing the pressure of the fuel pressurizing means when the load detected by the external load detecting means is lower than a prescribed value.
According to the fuel injection control apparatus for an internal combustion engine, the external load detecting means detects the load currently applied to the engine from the outside, and the detected value is output to the pressure modifying means. The pressure modifying means, based on the value detected by the external load detecting means, sets the pressure applied by the fuel pressurizing means to the fuel injected from the fuel injecting means. The fuel injection means then injects the fuel having been pressurized by the set pressure. When the external load detecting means detects that the load externally applied to the engine is not greater than the prescribed value, the pressure modifying means controls to lower the pressure applied by the fuel pressurizing means, so that the pressure of the fuel supplied to the fuel injection means is lowered. As the fuel pressure decreases, the valve opening speed of the fuel injection means is accelerated compared to the case where the fuel pressure is high. Thus, it is possible to shorten the time required for the valve to fully open, and stabilize the quantity of the fuel injected in the short injection period.
Of the injectors, the in-cylinder injector is attached to the cylinder head with its injection hole open to the combustion chamber. This means that the part near the injection hole is likely to suffer a high temperature compared to the intake manifold injector. When the part of the in-cylinder injector near the injection hole suffers a high temperature, needle, sheet and others will expand, resulting in the fuel injection quantity less than its target value. This problem is more serious at the time when fuel injection is being conducted only through the intake manifold injector, since the fuel within the in-cylinder injector hardly moves and the fuel and the in-cylinder injector both suffer a high temperature.
Thus, immediately after switching from the fuel injection through the intake manifold injector to the fuel injection through the in-cylinder injector, the fuel injection quantity becomes less than its target value, even though temporarily, due to the expansion of the in-cylinder injector. This inevitably leads to a leaner air-fuel ratio.
Meanwhile, in the fuel injection control apparatus for an internal combustion engine described in Japanese Patent Laying-Open No. 09-021369, the fuel pressure is decreased during the low load operation, which is considered to decrease noise and vibration (NV) of the fuel-supply system attributable to high pressure. Further, in the low load operation, the quantity of the fuel injected from the in-cylinder injector may be decreased, or fuel injection from the in-cylinder injector may even be stopped. The fuel may be injected only through the intake manifold injector ensuring more uniform fuel supply, or the quantity of the fuel injected from the intake manifold injector may be increased.
If the quantity of the fuel injected from the in-cylinder injector is decreased or fuel injection therefrom is even stopped, however, the temperature at the tip end of the in-cylinder injector increases, because it is not cooled by the fuel passing therethrough. In such a case, the temperature of the injection hole at the tip end of the in-cylinder injector will increase, and deposits will gradually block the hole.